The NiO/NiS composite catalyst synthesis is designed using a simple and facile method. Composite materials with different sulfur sources were characterized and applied for catalytic reduction of 4-nitrophenol (4-NP) in aqueous solution. The catalysts' universality towards reduction/decolorization of other organic dyes such as methylene blue (MB), methyl orange (MO), and rhodamine B (RhB) was evaluated. The NiO/NiS (Ni-5) composite catalyst prepared with 10 mmol of Ni(Ac)₂·4H₂O and 5 mmol of thioacetamide had the best catalytic activity and the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) took only 6 min. The composite catalyst also performed very well for reduction of MB, MO, RhB, and a mixture of organic pollutants including 4-NP, MB, MO, and RhB, which were completed in 60, 30, 90, and 210 s, respectively. The results showed that the Ni-5 composite catalyst was highly active and efficient for complete reduction of organic pollutants at room temperature. The catalytic efficiency of the composite material originates from NiS as a catalyst and facilitates electron transfer together with the NiO hierarchical structure. The pollutants and the hydrogen adsorbed on the surface of the catalyst can be combined with each other so that reduction occurs easily. Therefore, the NiO/NiS composite material formed from low-cost nickel-based metal oxide and sulfide can be successfully applied towards the reduction of organic pollutants at room temperature.